1. Field of the Invention
The present invention relates to a sheet conveying apparatus used in a printer.
2. Description of the Related Art
There has been an increasing demand for printers with high print quality and a demand for ever increasing precision. At the same time, there has been an increasing demand for cost reduction. Thus, both higher precision and cost reduction are expected for a printer. To address the situation, attempts have been made to detect the movement of a sheet being conveyed by taking an image of a surface of the sheet with an image sensor and performing image processing on the image, so as to detect the movement of the sheet with high precision and thereby convey the sheet under feedback control.
U.S. Pat. Nos. 7,104,710 and 6,599,042 disclose techniques related to the movement detection of the sheet. With the techniques, images of a surface of a sheet being moved are sequentially taken a plurality of times by an image sensor, the images are compared with each other by performing pattern matching, and the amount of the movement of the sheet is detected from the amount of difference between the images.
A sheet being conveyed may rise above a proper position and the distance between the image sensor and a surface of the sheet may change. Moreover, the distance between the image sensor and the surface of the sheet may change when the thickness of the sheet being used changes. If this phenomenon occurs, the size of an image taken by the image sensor is changed, so that the amount of the movement of the sheet may not be accurately detected. This can be prevented by using a telecentric lens or a lens having a focusing function as an imaging optical system. However, such an optical system is not suitable for reducing the cost and the size of the apparatus, because such an optical system is generally complicated and expensive.
When dust adheres to a surface of a lens of the imaging optical system or a surface of the image sensor, a clear image cannot be obtained, which may lead to erroneous detection. In particular, portions of ink drops ejected from the print head tend to float in an inkjet printer as ink mist, and the ink mist may adhere to the surface or a lens or an image sensor.
The present invention provides an improved apparatus in view of the above-described situation. In particular, the present invention provides an apparatus with which, by using image processing, the movement of a sheet can be reliably detected while preventing increase in cost. The present invention also provides an apparatus in which the detection accuracy does not decrease for a long time by preventing ink mist and dust, which float in the apparatus, from adhering to a sensor unit.